Semiconductor etching process control

ABSTRACT

The thickness of a silicon wafer ( 3 ) within a processing vacuum enclosure ( 1 ) is measured or monitored by an optical apparatus ( 50 ) via a window ( 4 ). The optical apparatus ( 5 ) comprises a laser which is tuneable across a range of wavelengths while maintaining a narrow bandwidth. The optical apparatus ( 5 ) also includes a detector receiving reflected light. The wavelength variation produces interference effects which are used, by examination of the detector output, to give a measure of thickness or other parameters.

The present invention relates to a method and apparatus for theinspection or monitoring of thin films, and to improved process controlusing the same in the production of thin film articles such asintegrated circuits.

Wavelength scanning interferometry is a known technique, in which asample is examined using a light beam whose wavelength is scanned acrossa range of wavelengths. In the prior art, however, light sources havebeen used which are wide-band and relatively broad in line width. Thishas meant that long signal processing times have been required, andwavelength scanning interferometry has not been suitable for use inon-line process control.

It is well known that monochromatic light impinging upon the smoothsurface of a material which is substantially transparent at thewavelength of such light, where either a) the back surface of saidmaterial is also smooth and substantially parallel to the front, or b)internal structures that present steps in refractive index are present,a multiplicity of reflected beams will arise and constructive anddestructive interference will occur between them. Where the thickness ofthe material intervening the various points of reflection remains fixed,and the wavelength of said monochromatic light is varied over a range ofvalues, and the coherence length of the source is greater than or equalto the optical path length difference between the various reflectionpoints, then the combination of the multiplicity of beams will move fromconstructive to destructive interference and back as the wavelength ofthe monochromatic light source changes. By monitoring the intensity ofthe combined beam as it changes with the wavelength of the source, it ispossible to determine the thickness of the layers that separate thevarious sources of reflection. Furthermore, by considering the imaginarycomponent of the refractive index of the various components of thematerial, it is possible to deduce chemical composition of some of thosecomponents.

It is well known that under certain specific conditions the control ofprocesses for etching materials from a surface or, alternatively,depositing material on that surface, can be conveniently arranged by ameans that follows the variation in intensity over time of a beam ofmonochromatic light reflected from said surface. Ref: FR-2718231).

The necessary conditions for such a means to function are:

-   -   That the surface undergoing etching or deposition provides, in        combination with the lower surface or the intervening layered        structures, a multiplicity of reflected beams, with each source        of reflection separated from each of the others by a distance        that is less than or equal to the coherence length of the        illuminating source; “coherence length” referring to the        coherent nature of the source and being related to the        reciprocal of the line width.    -   That the reflecting surfaces and or internal layer interfaces        are smooth and do not scatter more than a small percentage (less        than 5%) of the light incident upon them.    -   That the reflecting surfaces and or internal layer interfaces        are flat to better than {fraction (1/10)} of the wavelength of        the illuminating light over the area illuminated.    -   That the absorption by the material at the surface and at all        underlying structures at the wavelength of the illuminating        light is small (less than 10%).

If the reflection signal is tracked as material is etched from ordeposited onto the surface, then processing that signal with filtersderived from a mathematical model that simulates reflection from such asurface will provide for real-time process control of thickness etchedor deposited. (Ref: U.S. 6,226,086 B1)

The above method is applicable only to those instances where thethickness of the measured system varies by agency of either a depositionor etch process. It is limited to determination of one physicalparameter, the thickness of the changing layer, the values of the otherparameters having been input as pre-assumed values into the mathematicalmodel.

In conventional semiconductor (e.g. silicon) etch processes, a speciallayer is deposited which is later used as an etch-stop. A subsequentlayer is then deposited, which later will be etched down to this speciallayer. The etch process is a combination of chemical and physicalprocesses whereby the layer of interest is etched. The end point of theetch normally relies on a change in the chemical conditions in theplasma when the etched material is sufficiently eroded, exposing theetch-stop material. The etch chemicals react slower and with slightlydifferent chemistry with the etch-stop material. This change is detectedusing a variety of methods.

It would be beneficial to eliminate the entire process step used todeposit the etch stop layer. If an entire process deposition step (andassociated clean up steps) are removed, wafer throughput could beincreased as well as making a small positive impact on wafer yield. Inaddition the inclusion of the etch stop layer is often detrimental tothe performance of the completed semiconductor device which would havehad a superior performance if the etch stop layer had not been included.

It is an objective of the current invention to improve on this priorart.

Embodiments of the invention may provide one or more of the followingadvantages:

-   -   Measurement when the thickness of the measured system of films        is not varying.    -   Measurement of more than one parameter. For example, without        prejudice to the generality of this method, it would be possible        to determine the thicknesses of several layers in a multilayer        stack, or the thickness and chemical composition of a given        layer.    -   Man increase in speed and accuracy of mathematical manipulation        of data so that the invention may be used in real-time        applications as opposed to off-line use.    -   Measurement to detect etch endpoint without the use of an etch        stop layer even if the criteria for the endpoint is the        remaining thickness of a layer of the same composition which has        up to that point already been etched.

Accordingly, the present invention provides a method for inspection ormeasurement of thin films, in which the film is illuminated with a lightbeam, the wavelength of which is selected to be one at which the layerof interest is not absorbing, said wavelength is scanned through a rangeof wavelengths, and the intensity variation of the reflected beam ismeasured; and in which the light beam is derived from a light source ofvery narrow line width, the accuracy of the wavelength is maintainedwithin tightly defined limits, and the wavelength is tuned across thedesired range to derive a data set of reflection level and wavelength.

From another aspect the invention provides a method of etching a wafer,comprising positioning the wafer within a vacuum enclosure, measuringthe initial thickness of a desired point on the wafer by the method ofthe preceding paragraph, initiating an etching process, monitoring thethickness of said desired point by the method of the preceding paragraphas the etching progresses, and terminating etching when a desiredthickness is reached.

From a further aspect the invention provides apparatus for inspection ormeasurement of thin films, comprising a tuneable narrow band lightsource with a width of wavelength, which light source can be tunedacross a range of wavelengths while maintaining a narrow line width, andan optical assembly for focussing the laser spot on the film structureto be inspected and for transmitting reflected light to an opticalsensor.

Other features and advantages of the present invention will be apparentfrom the claims and from the following description, given by way ofexample only, of embodiments of the invention.

In the drawings:

FIG. 1 is a schematic cross-sectional view of a vacuum processing systemused in the production of integrated circuits;

FIG. 2 is a diagrammatic representation showing in more detail anoptical apparatus used in the system of FIG. 1.

FIG. 1 shows atypical vacuum processing vessel 1 containing twoelectrodes 2 for the generation of an electric field and in which asubstrate 3 to be etched is placed on the grounded electrode. A plasmais then produced between the electrodes 2 and a reagent gas introduced.The plasma dissociates the gas into the ions and radicals which bringabout the etching of the substrate 3. A window 4 is provided in thevessel 1, through which a laser beam is projected and the return beamreceived by an optical apparatus 5.

FIG. 2 shows, in diagrammatic form, the makeup of the optical apparatus.

An optical window 10 provides for the passage of light into and out ofthe optical assembly 5. A lens 9 provides for focussing the probe lighton to the film structure being measured and, at the same time, for relayof an image of that focussed spot and the adjacent surface on to animaging means 13. An additional illumination source 12 may be providedand introduced into the optical path by a beamsplitter 11 so that theadjacent surface may be readily detected by the imaging means 13 undercircumstances of low ambient illumination.

A laser 6 is provided having a wavelength that is substantially notabsorbed by the film structure being measured. Without loss ofgenerality, the film structure may be a silicon wafer with both surfacespolished and a starting thickness of 0.6 mm. Under those circumstancesthe laser 6 may be chosen to have a centre wavelength of 1550 nm, awavelength accuracy of +/−40 picometres, a linewidth of less than 10pico metres and a tuneable range of 100 nm. In general terms, the rangeof tuning should be such as to provide at least two turning points(maximum or minimum) as the wavelength is tuned across the range.

The laser is an Indium Phosphide semiconductor laser device operating ina single mode of operation and constrained to a particular wavelength byproviding external reflectance and wavelength selection means withprovision to smoothly and continuously adjust the same so that thecentre wavelength of illumination has a full width at half maximum of 10pico metres or less. Such lasers are commercially available.

Radiation from the laser 6 is introduced into the optical path of theimaging means 13 by a beamsplitter 8. After transmission to thesubstrate with its film system to be measured, the returned radiationpasses again to the beamsplitter 8 and part of the radiation then passesto a further beamsplitter 7 and is directed on to a detector 14. Thedetector 14 may be conveniently a high speed Gallium Indium Arsenidephotodiode.

Part of the illumination originating from the laser 6 after reflectionfrom the film system to be measured proceeds through the beamsplitter 8to form part of the image detected by imaging means 13.

The measurement is made by varying the wavelength of the laser 6 and atthe same time recording the signal from the detector 14. This data setis then input to a genetic algorithm means which may be convenientlyimplemented on a personal computer 15. An additional input to thegenetic algorithm means is a prediction of the boundary conditions ofthe film system that is being measured, this may be understood as thereis a priori knowledge that the film system parameters fall within thesedefined boundaries.

The function of the genetic algorithm means is then to produce candidatesolutions by reference to the mathematical description of the materialstructure under analysis. These candidate solutions are scored relativeto their closeness of behaviour to the data set obtained. The offspringcandidates from those solutions which are close to the data set survive,the offspring candidates from those solutions which are further from thedata set fail. In this way, by mathematically mimicking the principlesof natural selection a prime candidate is quickly and convenientlyfound. The parameters of the film system to be measured that arise fromthis efficient and convenient genetic algorithm processing means arethen conveyed by a data link 16 to a system control computer 17 thusproviding a means of on-line process control.

In one example of a suitable genetic algorithm, the algorithm employs athree-gene chromosome of which a first gene maps to the thickness of thethin film being measured or inspected, a second gene acts as amultiplier for the reflectance signal. and the third gene acts as anoffset modifier for the reflectance signal. This may be used to matchthe measured reflectance data set to that predicted by a mathematicalmodel of a single layer film of uniform and predetermined refractiveindex, the reflectance signal arising from a combination of thereflections from its upper and lower surfaces.

Alternatively, this algorithm may be used to match the measuredreflectance data set to that predicted by a mathematical model of amulti-layer structure, the outermost layer of which is of a uniform andpredetermined refractive index and forms the layer whose thickness is tobe measured, the measured reflectance signal being a combination of thatarising from any of the boundaries between layers in addition to thatarising from the upper surface of the structure, and which may be neednot include the contribution from the bottom surface.

This algorithm may also be used in the case where the layer to bemeasured does not have a uniform refractive index, but exhibits a knowngradient in refractive index which can be used in the mathematicalmodel.

In another example, the genetic algorithm uses a three-gene chromosomein which a first gene maps to the thickness of the thin film beingmeasured or inspected, a second gene maps to the refractive index ofthis film, and the third gene acts as an offset modifier for thereflectance signal.

Using either form of algorithm it is possible to ascertain the thicknessof a single film, or of the outermost layer of a multilayer structure,in the absence of a priori knowledge of its refractive index.

The foregoing apparatus and method can be used to control etching ofthin films without use of an etch stop layer. The light dot is focusedon an exposed surface of the silicon wafer to be etched. The laserwavelength is adjusted and the resulting interference pattern isanalysed to determine the thickness of the material. Etching then takesplace, during which the interference pattern will shift. The etching isstopped when the detected thickness corresponds to the originalthickness less the desired etch.

It will be understood that modifications may be made to the foregoingspecific examples within the scope of the invention.

The invention may be applied to the processing of materials other thansilicon, especially semiconductor material such as gallium-arsenide,silicon-germanium, germanium, indium phosphide. More than one chemicalmay be involved in the etching process, including inert materials. Thealgorithm may be implemented on any suitable device other than apersonal computer, such as a microcontroller or other embedded computingsystem.

It will be appreciated that the invention may be applied to themeasurement of non-varying structures, and to deposition as well asetching. It may be applied to forms of etching other than with chemicalvapours, for example ion beam etching, and to chemical-mechanicalpolishing using slurries and purely mechanical polishing.

1. A method for inspection or measurement of thin films, in which thefilm is illuminated with a light beam, the wavelength of which isselected to be one at which the layer of interest is not absorbing, saidwavelength is scanned through a range of wavelengths, and the intensityvariation of the reflected beam is measured; and in which the light beamis derived from a light source of very narrow line width, the accuracyof the wavelength is maintained within tightly defined limits, and thewavelength is tuned across the desired range to derive a data set ofreflection level and wavelength.
 2. The method of claim 1, in which saidlight beam has a line width at any point in time of less than or equalto 10 pico metres.
 3. The method of claim 1 in which said limits ofaccuracy are +/−40 picometres of the desired centre wavelength acrossthe range.
 4. The method of claim 1, in which the range is such as toprovide at least one interference maximum and minimum as the wavelengthis tuned across the range.
 5. The method of claim 4, in which the rangeis such as to provide two interference maxima and one minimum, or twointerference minima and one maximum.
 6. The method according to claim 1,in which the wavelength across the range is chosen such that less than10% of the light is absorbed by the material being measured.
 7. Themethod of claim 1, in which the measured intensity is processed byproviding a mathematical description of the film system being measured,said mathematical description defining boundaries of physical andchemical variation within which the system is known to fall at the timeof measurement.
 8. The method of claim 7, including determining apreferred match of potential solutions within said boundaries to saiddata set by applying a genetic algorithm.
 9. The method of claim 8 inwhich the genetic algorithm employs a three gene chromosome.
 10. Themethod of claim 9 in which a first gene maps to the thickness of thefilm being inspected or measured, a second gene acts as a multiplier forthe reflectance signal, and the third gene acts as an offset modifierfor the reflectance signal.
 11. The method of claim 9, in which a firstgene maps to the thickness of the thin film being inspected or measured,a second gene maps to the refractive index of said film, and the thirdgene acts as an offset modifier for the reflectance signal.
 12. Themethod of claim 8, in which the preferred solution is used as the inputto a real-time process control.
 13. The method of claim 12, in which theprocess to be controlled is one in which the measured film structure isvaried by a process selected from dry plasma etch, ion bombardment etch,film growth by physical vapor deposition, material removal by chemicalmechanical polishing, and material removal by mechanical polishing. 14.A method of etching a wafer, comprising positioning the wafer within avacuum enclosure, measuring the initial thickness of a desired point onthe wafer by the method of claim 1, initiating an etching process,monitoring the thickness of said desired point by the method of claim 1as the etching progresses, and terminating etching when a desiredthickness is reached.
 15. The method of claim 14, in which the wafer hasone or more areas to be etched which are not provided with a chemicallydistinct etch stop layer.
 16. The method of claim 1 in which the lightsource is derived from a Indium Phosphide semiconductor laser deviceoperating in a single mode of operation and constrained to a particularwavelength by providing external reflectance and wavelength selectionmeans with provision to smoothly and continuously adjust the same, thecentre wavelength of illumination having a full width at half maximum of10 pico metres or less.
 17. Apparatus for inspection or measurement ofthin films, comprising a tuneable narrow band light source with a widthof wavelength, which light source can be tuned across a range ofwavelengths while maintaining a narrow line width, and an opticalassembly for focussing the laser spot on the film structure to beinspected and for transmitting reflected light to an optical sensor. 18.Apparatus according to claim 17, in which the width of wavelength at anypoint in time is less than or equal to 10 pico metres.
 19. Apparatusaccording to claim 17, in which said light source is a single modelaser.
 20. Apparatus according to claim 19, in which the laserwavelength has an accuracy of+/−40 picometres of the desired centrewavelength across said range.
 21. Apparatus according to claim 17, incombination with computing means connected to receive the output of theoptical sensor, the computer means being operable to process the sensoroutput by use of a genetic algorithm.
 22. A material processing systemwhich comprises a vacuum enclosure, means within the enclosure forperforming etching or deposition on a wafer positioned in the chamber,and apparatus in accordance with claim 17; the apparatus beingpositioned exteriorly of the enclosure with said laser beam andreflection passing via a window in the enclosure wall.